1. Field of the Invention
The present invention generally relates to conventional piezoelectric transducers of the type which include a brass substrate having predetermined dimensions and a piezoelectric ceramic element having predetermined dimensions mechanically and electrically coupled to the brass substrate wherein the piezoelectric transducer has a fundamental resonant frequency and a fundamental nodal diameter. More specifically, the present invention relates to a method and means for reducing the resonant frequency of the piezoelectric transducer while maintaining the fundamental nodal diameter and substrate predetermined diameter.
Generally speaking, the novel method and means for reducing the resonant frequency of a cnventional piezoelectric transducer as described hereinabove includes the step of radially slotting the brass substrate of the transducer.
2. Description of the Prior Art
A piezoelectric transducer such as the one shown in FIGS. 1 and 2 has previously been used in audible alarm devices (See U.S. Pat. No. 3,815,129 assigned to P. R. Mallory & Co. Inc.) and has typically been operated at an audible frequency of about 3.0 KHZ which is substantially the fundamental resonant frequency of the transducer. In general, the prior art transducer includes a piezoelectric ceramic element mechanically and electrically coupled to a substrate, and at least two electrodes carried by the piezoelectric ceramic element. In order to attain an audible signal having a frequency of about 3.0 KHZ representing the fundamental resonant frequency of the transducer, the transducer will have certain predetermined dimensions, i.e., substrate diameter, substrate thickness, ceramic diameter, and ceramic thickness. Predicated upon these predetermined dimensions, the transducer will also have, in addition to a fundamental resonant frequency, a fundamental nodal diameter. Typically such transducers are mounted at at least one point on the circumference of a circle having a diameter substantially equal to the fundamental nodal diameter.
It has become desirable to reduce the fundamental resonant frequency of a conventional piezoelectric transducer as described hereinabove while maintaining the same fundamental nodal diameter and substrate diameter so that an audible alarm device having a lower audible frequency can be provided in the same packaging as the higher 3.0 KHZ audible alarm device. As known to those skilled in the art, the typical methods for reducing the fundamental resonant frequency of a free circular disk include increasing the diameter of the disk, changing the material composition of the disk, or reducing the thickness of the disk. However, to increase the substrate diameter of the above described transducer would result in a corresponding increase in the fundamental nodal diameter of the transducer, and materials which are as economic to use as the materials comprising the conventional 3.0 KHZ transducer do not provide any substantially significant advantages. Furthermore, the seemingly only other approach of reducing the thicknesses of the piezoelectric ceramic element and/or the substrate is not practical because of the limited ability to economically manufacture a piezoelectric ceramic element having a thickness significantly less than its predetermined thickness.